1. Field
The disclosed method and apparatus relates to wireless communications, and more specifically to wireless systems that employ signal acquisition assistance data to assist a receiving station in acquiring selected signals.
2. Related Art
The wireless communications industry is developing services that generate accurate position information for wireless terminals. This development is partly motivated by the needs of public safety service providers in their efforts to promptly respond to emergency calls. In many instances, the calling party may be unwilling or unable to provide accurate position information. When such information is provided automatically, public safety officials are able to respond and render service quickly. In general, a location at which a public safety entity receives emergency ‘911’ telephone calls is known as the Public Safety Answering Point (hereinafter ‘PSAP’).
The well-known Global Positioning System (GPS) offers one possible approach to providing wireless terminal position determination. Using GPS technology, wireless terminal position and velocity information can be determined within the accuracy requirements required by the FCC report and order. In addition to providing sufficiently accurate wireless terminal position information, new GPS features are easily integrated into a wireless telephone once GPS technology is added to the unit. The extra value features can be used to increase the market value of the wireless phone and to enhance revenues through the provision of additional services to the end users of such telephones.
As is well known, the GPS navigation system employs satellites that are in orbit around the earth. Any user of GPS, anywhere on earth, can derive precise navigation information including 3-dimensional position, velocity and time of day. The GPS system includes 24 satellites that are deployed in circular orbits with radii of 26,600 kilometers in three planes inclined at 55° with respect to the equator and spaced 120° with respect to one another. Eight satellites are equally spaced within each of the three orbit paths. Position measurements using GPS are based on measurements of propagation delay times of GPS signals broadcast from the orbiting satellites to a GPS receiver. Normally, reception of signals from 4 satellites is required for precise position determination in 4 dimensions (latitude, longitude, altitude, and time). After a receiver measures respective signal propagation delays, a range to each satellite is calculated by multiplying each delay by the speed of light. The location and time are found by solving a set of four equations with four unknowns incorporating the measured ranges and the known locations of the satellites. The precise capabilities of the GPS system are maintained using on-board atomic clocks for each satellite, in conjunction with tracking stations that continuously monitor and correct satellite clock and orbit parameters.
Each GPS satellite vehicle (SV) transmits two direct-sequence-coded spread spectrum signals in the L-band: an L1 signal at a carrier frequency of 1.57542 GHz, and an L2 signal at 1.2276 GHz. The L1 signal consists of two phase-shift keyed (PSK) spread spectrum signals modulated in phase quadrature: the P-code signal (P for “precise”), and the C/A-code signal (C/A for “coarse/acquisition”). The L2 signal contains only the P-code signal. The P and C/A codes are repetitive pseudo-random (also called pseudo-noise or “PN”) sequences of bits (termed “chips” by those skilled in the telecommunication arts) that are modulated onto the carriers. The clock-like nature of these codes is utilized by the receiver in making time delay measurements. The PN codes for each SV are unique, allowing the receiver to distinguish which satellite transmits a given code, even though they are all transmitted at the same carrier frequency. A 50 bit/sec data stream containing system status information and satellite orbit parameters, useful for the navigation calculations, are also modulated onto each carrier. The P-code signals are encrypted, and are not generally available for commercial and private users. The C/A signal is available to all users.
The operations performed by GPS receivers are, for the most part, typical of those performed by direct-sequence spread spectrum receivers. The spreading effect of the PN code modulation must be removed from each signal by multiplying it by a time-aligned, locally generated copy of the code, in a process known as “despreading.” Because the appropriate time alignment, or code delay, is unlikely to be known at receiver start-up, it must be determined by searching during the initial “acquisition” phase of GPS receiver operation.
After despreading is performed, each signal consists of a 50 bit/sec PSK signal at an intermediate carrier frequency. The exact frequency of this PSK signal is uncertain due to the Doppler effect caused by relative movement between the satellite and the terminal unit, and due to local receiver GPS clock reference errors. A search for the Doppler frequency must be performed during initial signal acquisition, because it is usually unknown prior to signal acquisition. Carrier demodulation can proceed once the Doppler frequency is approximately determined.
After performing carrier demodulation, data bit timing is derived using a bit synchronization loop, and the data stream is finally detected. A navigation calculation may be undertaken once the signals from four satellites have been acquired and locked onto, the time delay and Doppler measurements have been made, and a sufficient number of data bits (enough to determine the GPS timing reference and orbit parameters) are received.
One disadvantage of the GPS system for location determination is the relatively long time needed to perform signal acquisition. As noted above, SV signals cannot be tracked until they have first been located by searching in a two-dimensional search “space”, whose dimensions are code-phase delay and Doppler frequency shift. Typically, if there is no prior knowledge of a signal's location within this search space, as would be the case after a receiver “cold start”, a large number of code delays (about 2000) and Doppler frequencies (about 15) must be searched for each SV signal that is to be acquired and tracked. Thus, for each signal, up to 30,000 locations in the search space must be examined. Typically these locations are examined sequentially, one at a time, a process that can take as long as 5 to 10 minutes. The acquisition time is further lengthened if the identities (i.e., PN-codes) of the four satellites within view of the receiving antenna are unknown.
Signal acquisition is needed at least when a GPS receiver has lost the signals, which may occur, for example, after power down, or when the signal has been blocked from the receiver for some period of time. After acquiring the signals, they may be maintained or “tracked.”
However, many devices such as cellular telephone mobile stations (MSs) possess GPS functionality as an additional feature or enhancement, rather than as a primary purpose of the device. For these devices, a need to continuously track GPS SV signals would increase the cost, decrease the battery life, or reduce the functionality of the primary device (e.g., primarily functioning as a cell phone). For example, because GPS SV signals are provided at frequencies that differ from cellular telephone signal frequencies, a single receiver cannot simultaneously monitor both frequencies. In order to do so, an MS would need an additional receiver unit, thereby adding to the cost of the device. Moreover, the processing capability of the system would need to be increased in order to concurrently monitor both signals, which would increase both cost and power consumption. Accordingly, many such systems rarely track GPS SV signals, but rather acquire the needed signals only upon demand.
All GPS-capable systems require acquisition of GPS SV signals. Some systems only occasionally require such acquisition, while others require acquisition of the GPS SV signals each time they are needed for a GPS function. The need for signal acquisition, unfortunately, does not prevent GPS functions from being needed urgently, such as when an MS location is required quickly to facilitate response to an emergency. In such situations, the time delay associated with a 5 to 10 minute GPS satellite signal acquisition cold-start by a GPS/wireless terminal unit before a position determination can be obtained is highly undesirable.
In order to reduce this delay, information may be provided to aid a receiver in acquiring a particular signal. Such acquisition assistance information permits a receiver to narrow the space that must be searched in order to locate a signal, by providing a “code window.” The code window provides a reduced range within which the “code phase” (effectively, the signal time of arrival) should be found, or a predicted range of Doppler shift associated with the signal. Acquisition assistance may also include other information about the signal, such as its PN (pseudo-noise or pseudo-random) code, frequency, modulation, and content. The narrower the windows on the uncertainties of the signal are, the more quickly the receiver can acquire the signal. Narrowing these windows not only enables faster acquisition of signals, which shortens the delay before a location determination can be produced, but also reduces the processing burden on the receiver, which may reduce power consumption. Systems in which receivers locate ranging signals for position location (such as SV GPS signals) upon demand, with the assistance of information provided from another source within the system, are generally referred to as “wireless assisted position location” systems.
Since their introduction, wireless assisted position location systems have been gaining popularity as the preferred position location technology. They are applicable in any system where user terminals capable of measuring ranging signals from given signal sources can access a data base in order to obtain information facilitating quick acquisition of the ranging signals. Among others, one of the applications is use by position location capable wireless mobile stations (MSs) communicating with one or more base stations (BSs), where the BSs are connected to one or more data base servers, also called Position Determination Entities (PDEs), which can provide signal acquisition assistance data.
Signal acquisition assistance information transmitted to an MS may include description of the type of ranging signals available to the MS and characterization of those signals, such as frequency, PN offset, expected code phase, etc. Determination of some of these parameters is based on an approximate knowledge of the location and the internal state of the MS. A goal of such acquisition assistance information is to permit the MS to predict the time of arrival, or code phase, of a particular SV signal, and the Doppler shift of the SV signal if applicable, which may be referred to generally as code phase prediction. Considerations include the generation, conveyance, and use of code phase prediction information.
Code phase prediction is only as accurate as the parameters input for its calculation. Because the clock offset, position and movement of an MS relative to a source of a particular signal that is desired are generally not precisely known, the predicted code phase has some uncertainty, which can be expressed as an uncertainty window around a predicted code phase and an uncertainty window around a predicted Doppler shift of the signal.
Wireless assisted position location systems suffer from a drawback of latency due to their need to receive acquisition assistance information when, for instance, GPS functions are required. The latency is due not only to the time required to acquire the requisite signals, but also the time required to request acquisition assistance information, generally from another entity within the telecommunications system, for that entity to collect and provide the acquisition assistance, and for the acquisition assistance to be received. Moreover, communicating the acquisition assistance information burdens the communication system carrying capacity.
Therefore, a need exists for a system and method that enable accurate code phase prediction using acquisition assistance, while reducing processing and communications burdens on receivers in wireless assisted position location systems.